Jan C.M. De Hoog

Ultramafic cumulates of oceanic affinity in an intracontinental subduction zone: ultrahigh-pressure garnet peridotites from Pohorje (Eastern Alps, Slovenia)

Publisher Summary Continental subduction and exhumation have been recognized as processes common to continental collision, which has led to the widespread occurrence of ultrahigh-pressure (UHP) metamorphosed rocks in collision belts. Garnet peridotites are subordinate but common constituents in nearly all UHP terranes. They are classified as crustal or mantle derived depending on their emplacement within the crust prior to or during continental subduction. The origin of peridotites is diverse and includes ultramafic cumulates and residual mantle of subcontinental, oceanic, or sub-arc mantle affinity. Identification of this origin is not always straightforward due to complex, often multiphase metamorphic histories, but these rocks may provide important information about the geodynamic and premetamorphic history of their host terranes. The shallow level of intrusion of the garnet peridotite protolith indicates that the SBUC mantle rocks were exhumed at the time of magma emplacement. Remnants of igneous minerals allow identification of the composition of melts from which cumulates crystallize. The rocks are of oceanic affinity and form the more primitive equivalent to associated eclogitized metagabbros within the SBUC and surrounding continental crust. The oceanic affinity of mafic and ultramafic units in Pohorje indicates a depleted asthenospheric mantle source.

First measurements of OH-C exchange and temperature-dependent partitioning of OH and halogens in the system apatite–silicate melt

Abstract We present the first integrated study of carbonate, hydroxyl, fluoride, and chloride ion partitioning in the apatite-melt system. We determined volatile partitioning behavior between apatite and silicate melt for both haplobasaltic andesite and trachyte bulk compositions at 0.5–1 GPa and 1250 °C using the piston-cylinder apparatus. All volatile species were analyzed directly in both apatite and glass using secondary ion mass spectrometry (SIMS) and electron probe microanalysis. Distribution coefficients for OH-halogen exchange are similar to those from previous studies, and together with literature data, reveal a significant log-linear relationship with temperature, while the effects of pressure and melt composition are minimal. Meanwhile, halogen-free experiments generate very high C contents (up to 5000 ppm) in apatite. Stoichiometry calculations and infrared spectra indicate that this C is mainly incorporated onto the channel volatile site together with hydroxyl. In halogen-bearing experiments, apatite crystals contain significantly lower C (≤500 ppm), which may be partly incorporated onto the phosphate site while the channel volatile site is filled by OH+F+Cl+C. Our experiments give the first constraints on H2O-CO2 exchange between apatite and silicate melt, with a KD of 0.355 ± 0.05 for the trachyte and 0.629 ± 0.08 for the haplobasaltic andesite. The new constraints on the temperature-dependence of partitioning will enable quantitative modeling of apatite-volatile exchange in igneous systems, while this new partitioning data and method for direct, in situ analysis of C in apatite mark a significant advance that will permit future studies of magmatic C and other volatiles. This has a broad range of potential applications including magmatic differentiation, fractionation, and degassing; quantification of volatile budgets in extraterrestrial and deep earth environments; and mineralization processes.

Boron Isotopes as a Tracer of Subduction Zone Processes

This chapter reviews recycling of boron (B) and its isotopes in subduction zones. It discusses the profound changes in B concentrations and B isotope ratios of various materials involved in convergent margin evolution, in particular highlighting the fate and evolution of progressively dehydrating subducting slabs and the behavior of B during burial and subsequent metamorphism. We review various models used to parameterize these complex and often poorly understood processes and critically evaluate the available data from the literature. We proceed by reviewing B isotope data from mafic arc volcanic rocks and explore systematic variations with subduction zone geometry as well as familiar geochemical tracers of subduction processes. Finally, the role of serpentinisation in the mantle wedge is discussed in the light of new geochemical and petrological insights on the importance of serpentinites and subduction erosion. We provide recommendations for further research on B isotopes in subduction zones and directions where we think this exciting stable isotope tracer may make the biggest impact.